Thermal stability of Group 6 bis(cyclopentadienyl) and ethylene bridged bis(cyclopentadienyl) monocarbonyl complexes; a theoretical study

Density functional calculations have been used to estimate the energy chang e involved in dissociation of CO from [M(eta-C5H5)(2)(CO)] (M=Cr, Mo or W) and [Cr{C2H4(eta-C5H4)(2)}(CO)] to give, in each case, free CO and a triple t metallocene product. The dissociation energy for [M eta(-C5H5)(2)(CO)] is calculated to be 99 (Cr), 186 (Mo) and 194 kJ mol(-1) (W) showing [Cr(eta- C5H5)(2)(CO)] to be significantly less thermally stable than the Mo or W an alogues consistent with the experimental findings. For the ansa-bridged com pound [Cr{C2H4(eta-C5H4)(2)}(CO)] the calculated dissociation energy was 11 2 kJ mol(-1), significantly more endothermic than for [Cr(eta-C5H5)(2)(CO)] , consistent with the enhanced thermal stability found for [Cr{C2Me4(eta-C5 H4)(2)}(CO)] as a result of introduction of an ansa-bridge. Theoretical ana lysis shows that the cause of the differences in thermal stability lies in the stability of the triplet metallocene product. The high spin-pairing ene rgy for Cr favours formation of a triplet state more than is the case for M o and W. Formation of a triplet state is of less advantage in the ansa-brid ged free chromocene as the rings are unable to relax to a near parallel str ucture, thereby lowering the energy of the d electrons. Such relaxation is prevented by the presence of an ansa-bridge.